Modulation modes: bandwidth of various signals; choice of emission type

T8A01

Which of the following is a form of amplitude modulation?

A.

Spread spectrum

B.

Packet radio

C.

Singlesideband

D.

Phase shift keying (PSK)

T8A01

Single Sideband is
a modulation mode commonly used with HF operation and can be essentially
described as a variant of amplitude modulation (AM) that only uses half of the
signal and no carrier. Because of this, it is easier to use over a long
distance where the reduced bandwidth usage results in less interference and
the lack of carrier makes it more likely that the signal will be copyable
despite any interference.

Spread-spectrum is not in and
of itself a form of modulation, but rather is a technique (or a family of
techniques, actually) that spreads a signal out on a wide range of frequencies
in order to reduce interference, avoid detection, etc.

Packet
Radio could probably be used with
Amplitude Modulation, but itself can be used with many different modulations.

What type of modulation is most commonly used for VHF packet radio transmissions?

A.

FM

B.

SSB

C.

AM

D.

PSK

T8A02

Since packet radio benefits from good bandwidth and minimal audio amplitude
variations, both of which are advantages of FM; and because FM is an allowed modulation method on
VHF, FM is the most commonly used modulation for packet.

Which type of voice mode is most often used for long-distance (weak signal) contacts on the VHF and UHF bands?

A.

FM

B.

DRM

C.

SSB

D.

PM

T8A03

The listed choices are: Frequency Modulation (FM), Single Side-Band (SSB),
DRM, and Phase Modulation (PM). All of these have their
own advantages and disadvantages, but of them Single Side-Band is unique in
that it does not transmit a carrier and requires approximately half of the
bandwidth of an FM signal; these two characteristics make it ideal for long-
distance and weak signal contacts in nearly any band because less of the
signal needs to make it through for the receiver to correctly copy the
transmission.

For comparison, consider your AM/FM car radio; when you are too far from the
tower, the signal begins to fuzz making it uncopyable. With SSB the signal
would fade but you would have less "white noise" in the faded signal and more
of it would be just the voice part that you are interested in. The downside to
this mode is that without a carrier, even when the signal is strong it may not
be as clear as an AM or FM signal.

Which type of modulation is most commonly used for VHF and UHF voice repeaters?

A.

AM

B.

SSB

C.

PSK

D.

FM

T8A04

FM (Frequency Modulation) is the same modulation used by the FM radio in your car, which makes it easier to remember. Note that regular FM radio broadcast stations (music, talk radio, etc) use what is commonly known as Wide FM, whereas ham radio generally uses Narrow FM which uses less bandwidth (about 5-15kHz).

CW or "Continuous Wave" is also known as "Morse
Code". This consists of turning the RF carrier "on" and "off". Since the signal only has to be strong enough to detect if the transmitter is on or not, it requires very little bandwidth.

What is an advantage of single sideband (SSB) over FM for voice transmissions?

A.

SSB signals are easier to tune

B.

SSB signals are less susceptible to interference

C.

SSB signals have narrowerbandwidth

D.

All of these choices are correct

T8A07

A rough way of understanding single sideband is to consider that AM voice
transmissions have two sidebands -- upper and lower, with each sideband being
approximately half of the bandwidth of the signal. With Single Sideband there
is no carrier and only one of the sidebands is used, either Upper or Lower,
and as a result SSB has a much narrower bandwidth.

It is true that SSB signals are more likely to be heard than FM signals when
the signal is weak, but this does not imply that they are less susceptible to
interference or that they are easier to tune -- they are simply still copyable
if a smaller portion of the signal makes it through.

What is the approximate bandwidth of a single sideband (SSB) voice signal?

A.

1 kHz

B.

3 kHz

C.

6 kHz

D.

15 kHz

T8A08

Single Sideband is
a form of modified Amplitude Modulation (AM). Whereas AM normally uses twice
the bandwidth of the original carrier, Single Sideband avoids that issue and
also does not waste power on a carrier.

The bandwidth used by a single sideband voice signal varries between 300 and
3400 Hz, or .3 to 3.4 kHz. The approximate bandwidth, therefore, is the rough
maximum used, which is approximately 3kHz.

This can be demonstrated on a 2 meter handheld radio if you have an interest
to see it; if you transmit on 146.520MHz and listen on 146.525MHz you will
likely still be able to hear the signal. If you can't (or barely can) then the
bandwidth is closer to 5kHz (standard for FRS radios, for example) and if you
can hear it strongly it may be closer to 10 or 15kHz. The further away you get
the wider the bandwidth would need to be for you to still be able to hear it.

Remember that if the bandwidth is 5kHz you will only hear them at the transmit
frequency +/- 2.5Khz, because the transmit frequency is the middle so half
will be above and half below. Most handheld radios seem to use 10kHz, but some
support "half deviation" mode which uses 5kHz.

What is the typical bandwidth of analog fast-scan TV transmissions on the 70 centimeter band?

A.

More than 10 MHz

B.

About 6 MHz

C.

About 3 MHz

D.

About 1 MHz

T8A10

Analog fast-scan TV transmissions use a lot of bandwidth compared to voice and other modes because of how much information is attached to the minorities that need to be transmitted. The approximate bandwidth is about 6 MHz.